This invention relates to an apparatus for the ion exchange chromatographic analysis of ionic species, wherein an eluant ion suppressor cell is employed. In another aspect, the invention relates to a method for the ion exchanged chromatographic analysis of ionic species.
In recent years, a form of chromatography called "ion chromatography" has developed. It has been used for the laboratory analysis of inorganic compounds suitably ionized in solution. Reference is made to U.S. Pat. No. 3,920,397 and other patents assigned to the Dow Chemical Company for descriptions of prior art ion chromatography systems.
In such prior ion chromatography systems, ionic species to be analyzed are introduced in solution to a separation column which contains an ion exchange resin. Also introduced to the separation column is a flow of eluant solution which acts as a carrier for the ionic species. Within the resin bed, ions in the eluant solution act to displace ionic species ions which have become bound to the resin bed at ion exchange sites. The separation column discharges an effluent which contains separated ionic species. As used herein, "separated" means that the ionic species appear at different times in the effluent, i.e., the species are resolved. In addition to the separated ionic species, however, the effluent also contains eluant solution ions. These eluant ions act essentially as background noise, and tend to cover up the ionic species of interest, making the ionic species barely detectable. Thus, a suppressor column is typically included to neutralize the eluant ions. The suppressor column also includes an ion exchange resin bed which neutralizes the eluant ions so as to convert the eluant solution to weakly ionized form. The fluid discharged from the suppressor column is then passed to a suitable detector, such as a conductivity detector, which detects the ionic species.
The most serious problem in the above described ion chromatography system resides in the suppressor column. The resin bed in the suppressor column after a period of time becomes "spent", and must be replaced or regenerated. This regeneration typically involves soaking the resin bed in a strong mineral acid or a strong base. Such periodic regeneration or replacement is time consuming and expensive.
Another problem associated with the suppressor column is its tendency to add to the "void" volume of the chromatography system. As used herein, void volume refers to the volume through which the separated ionic species must flow. Such void volume tends to spread the ions, thus resulting in band spreading or spreading of peaks obtained in the readout equipment. Each peak corresponds to a particular ionic species. As with any detection apparatus, it is always desirable to obtain the sharpest peaks possible. The suppressor column, therefore, tends to cause undesirable band spreading due to its significant addition to the void volume of the system.